1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine or laser beam printer which forms an image by transferring a toner image of at least one color onto a sheet by an electrophotographic method or electrostatic recording scheme, and an image forming apparatus control method.
2. Description of the Related Art
FIG. 18 shows an example of a conventional image forming apparatus.
The image forming apparatus comprises a rotary developing unit 3 rotatably supported by a rotation support (not shown). The rotary developing unit 3 includes a yellow toner developing unit 3Y, magenta toner developing unit 3M, cyan toner developing unit 3C, and black toner developing unit 3K.
The color toner developing units 3Y, 3M, 3C, and 3K of the rotary developing unit 3 sequentially face a photosensitive drum 4 to develop images with the respective color toners.
The photosensitive drum 4 serving as a photosensitive body is driven to rotate at a predetermined angular velocity, and the drum surface is uniformly charged by a charger 8. The drum surface is exposed and scanned with a laser beam in accordance with image data of the first color (e.g., yellow), forming an electrostatic latent image of the first color on the photosensitive drum 4. The yellow toner developing unit 3Y for the first color develops and visualizes the electrostatic latent image. The visualized first toner image is transferred onto an intermediate transfer member 5 driven to rotate in press contact with the photosensitive drum 4 at a predetermined press force.
This transfer process is similarly repeated for the remaining toners (magenta, cyan, and black). Toner images of the respective colors are sequentially transferred onto the intermediate transfer member 5, forming a color image. For a full-color print, color images transferred on the intermediate transfer member 5 are transferred at once onto a sheet 6 fed from a sheet feed unit. The sheet 6 bearing the color images is discharged after the fixing process by a fixing unit 7, obtaining a full-color print.
These days, as the number of full-color outputs increases, the stability of density of an output image and the stability of tonality are required of electrophotographic image forming apparatuses of this type.
In this situation, there is proposed an image density/tonality control method of stably maintaining density for a long period in electrophotographic image forming apparatuses such as a copying machine and printer.
According to this proposal, an image forming condition table corresponding to the environmental status and the durable number of sheets is stored in advance. The environment around the image forming apparatus is detected from an output from an environmental sensor incorporated in the image forming apparatus.
The durable number of sheets of the image forming apparatus or process unit is detected from a sheet counter incorporated in the main body. Appropriate image forming conditions are selected from the image forming condition table on the basis of the durable number of sheets.
According to this proposal, however, it is difficult to cope with a case where the state of the image forming apparatus deviates from the image forming condition table due to an unexpected use. A small change of the state of the image forming apparatus cannot be tracked.
To solve this, there is proposed the following technique. First, a density sensor detects the density of a specific toner patch formed on a photosensitive drum or transfer member. Then, image forming conditions are selected on the basis of the detected density. The image forming apparatus is controlled to obtain a predetermined density or tonality.
According to this proposal, the image forming apparatus can be controlled in accordance with its state, and a stable image can be obtained for a long period. A fine output image according to the state of the image forming apparatus can be attained by executing density/tonality control when the image forming apparatus starts up after left to stand for a long time, or every predetermined number of sheets.
Recently, the throughput needs to be maintained while stabilizing the density and tonality, in order to obtain a fine output image according to the state of the image forming apparatus. With this proposal, however, it is difficult to satisfy both the control frequency and maintenance of the throughput.
Density/tonality control is done by detecting not the density on a sheet but a pattern formed on the photosensitive drum or transfer member. Thus, a density obtained by control and an actual density on the sheet differ from each other.
To solve these problems, the following technique is proposed for tonality control in an image forming apparatus.
According to this proposal, an image reader reads a specific tone pattern formed on a sheet, determining a density correction characteristic. An optical sensor detects the density of an image formed on an image carrier such as the photosensitive drum in accordance with the density correction characteristic, storing the detection result.
The density correction characteristic is adjusted on the basis of the relationship between the stored detected density and the density, detected by the optical sensor, of an image formed on the image carrier at a predetermined timing (see, e.g., Japanese Patent No. 3441994).
In Japanese Patent No. 3441994, the density at each halftone level can be adjusted to a desired one by correcting the density correction characteristic on the basis of the relationship between the stored detected density and the detected density of an image formed on the image carrier at a predetermined timing. However, the maximum density cannot be adjusted to a desired one.
As for the maximum density, an image forming contrast potential is set as an image forming condition defined when the density correction characteristic is determined.
For example, even if the maximum density decreases upon the lapse of time after determining the density correction characteristic, it cannot be increased by the method of correcting the density correction characteristic (input signal) because there is no means for increasing the maximum density upon density variations.
When the optical sensor detects the density of a specific pattern formed on the image carrier such as the photosensitive drum, especially an optical sensor using specularly reflected light is lower in detection precision in the high-density region than in the low- and intermediate-density regions, and the detection value greatly varies.
For this reason, no high detection precision can be obtained when controlling the maximum density by forming a high-density pattern in solid black or the like on the image carrier and detecting it.
It is an object of the present invention to provide an image forming apparatus capable of maintaining the throughput, and maintaining a desired maximum density stably at high precision for a long period, and an image forming apparatus control method.